Sports footwear, in particular for sports such as running, basketball, tennis, volleyball, walking etc. comprises a sole unit associated with a vamp and is divisible substantially into two well defined parts, namely a lower part or treading sole making contact with the ground and generally of rubber or similar material, and an upper part or intersole overlying the lower part. The intersole is usually formed of expanded elastomers or mixtures thereof, such as ethylvinylacetate or polyurethane.
For many years it has been required that sports footwear of the aforesaid type be able to return to the user at least a part of the impact force which the user generates on the ground during his movement, so as to facilitate lifting of the foot and make this movement easier.
Besides the aforesaid characteristic, a particular requirement of such footwear is, inter alia, to optimize the position of the user's foot at the moment of contact with the ground in order to prevent deformation of the bone structure of the user's limb. To this must be added the requirement of facilitating the natural sequence of pronation and supination movements by the user.
Various types of sports footwear are known which attempt to satisfy the aforesaid requirements. From U.S. Pat. No. 5,092,060 and U.S. Pat. No. 5,369,896, sports footwear are known of the type comprising a vamp, a lower support part for said vamp, and a sole unit provided in this lower part. Inside the sole unit, within at least that part thereof positioned in correspondence with the user's heel, there is provided an insert comprising an airtight casing containing a plurality of elastically deformable elements substantially spaced apart in a uniform manner. These elements have substantially the same height and are barrel-shaped. A modification of this arrangement uses an insert of the aforesaid type also positioned in that part of the sole unit corresponding to the metatarsal regions of the user's foot. The elastically deformable elements are connected together by arms lying substantially in a horizontal plane and connected to the largest cross-section region of said elements.
Further footwear is known, forming the subject of a different patent in the name of the present applicant, comprising a sole unit of the aforesaid type provided with at least one insert having elastically deformable elements inserted into an airtight casing. These elements have different heights to give the casing upper surface facing the user's foot a concave shape, in order to provide optimum stability to the user's foot during movement.
In these arrangements and in others known in the state of the art, the deformable elements of the various inserts (or those provided within the sole unit) possess homogeneous mechanical characteristics. In other words, these elements all provide substantially the same elastic response to their stressing by the user's foot during his movement along the ground.
Other arrangements are known comprising a sole unit in which inserts are provided comprising a casing containing air or one or more fluids in general. These arrangements enable the response of the various parts of the insert to be differentiated according to their stressing during the movement of the user and enable different damping characteristics to be obtained for the impact force generated by the user on the ground. These arrangements are, however, very costly and often do not achieve optimum differential response to stressing.